The nail plate (i.e., the natural nail) is primarily composed of keratin, a water-insoluble, fibrous protein that is a major structural component of skin, hair, wool, silk, feathers, scales, nails and hooves. While keratins can obviously differ greatly in their amino acid makeup, hard keratins may all be generally characterized as cross-linked polypeptides. Alpha-keratins such as nails and hooves may be further characterized by their relatively higher percentages of the amino acid cysteine. Typically, the alpha-helix coils of the polypeptides are cross-linked with disulphide bonds between adjacent cysteines. The resulting plate-like cells are cemented to each other with a sticky substance and held together by rivet-like structures called desmosomes. Many cell layers adhere to each other to form the nail plate, a structure that resembles a brick and mortar wall.
Conventional coatings for natural nails may be generally classified into three categories: nail polishes (also known as lacquers, varnish or enamels); artificial nails (also known as gels or acrylics); and hybrids. Nail enamels typically comprise various solid components which are dissolved and/or suspended in non-reactive solvents. Upon application and drying, the solids deposit on the nail surface as a clear, translucent or colored film. Usually, nail polishes are easily scratched and are easily removable with solvent, usually within one minute and if not removed as described, will chip or peel from the natural nail in one to five days.
Nail enamels coat the surface of the nail plate to provide a decorative finish with a characteristic glossy finish. Nail enamels conventionally comprise a film forming component, which is frequently nitrocellulose, cellulose acetate butyrate, or a combination of one or both of those cellulosics with a polyester or other polymeric compound. Most nail polishes are made of nitrocellulose dissolved in a solvent (e.g. butyl acetate or ethyl acetate) and either left clear or colored with various pigments. Typical components may include: film forming agents, resins and plasticizers, solvents, and coloring agents.
Artificial nails polymerize on the surface of a natural nail to form a hard, tough surface. Artificial nails conventionally include one or more (meth)acrylate monomers and a photoinitiator or hardener which may be mixed immediately before use. Optionally, the artificial nail composition may include a solvent or may utilize a liquid (meth)acrylate as a solvent. Artificial nails of this sort typically bond tightly and possibly irreversibly to the nail plate and must be removed by physical means such as filing.
Hybrid systems include both film-forming components and polymerizable components. In exemplary hybrid systems, the polymerizable components, for example (meth)acrylates, form a 3-dimensional (3-D) thermoset lattice and the film forming component, for example nitrocellulose or cellulose acetate butyrate is dispersed within the 3-D network. The 3-D thermoset lattice provides enhanced durability, toughness, and scratch-resistance over conventional nail enamels while the interdispered film-forming component provides a soluble network to allow for improved removability characteristics over artificial nails.
Application of nail coatings to the surface of the nail plate typically requires the surface of the nail plate to be treated. The surface treatment typically involves the use of a primer and/or roughening of the nail plate such as with the use of a file. This treatment process may cause damage to the nail plate, which is particularly problematic for individuals having thin nails.
Primers are adhesion promoters that improve adhesion by increasing interfacial compatibility between surfaces, e.g., the nail plate and an applied coating. For example, a coating of nail polish may resist chipping and peeling if a good primer is used. Primers are more compatible with the nail plate than the nail polish. Primers act as the “go-between” or “anchor”, to improve adhesion.
Primers are also frequently used with artificial nail enhancements since acrylic nail products normally have poor adhesion to nail plates. In general, nail plate primers can be thought of as double-sided sticky tape, joining the nail plate to the nail enhancement. The nail plate surface is made up of chemical groups possessing specific structures. Primer molecules typically match the chemical and structural characteristics of the nail plate. In specific embodiments, one end of the primer is reactive with methacrylate monomers. With these types of primers, physical abrasion of the nail plate is required to achieve proper levels of adhesion to the keratin substrate. As such, these primers can be destructive, and if used improperly they can cause damage to the nail plate and surrounding tissue. These primers can also cause discoloration of the nail enhancement. Indeed, they are the leading cause of discoloration.
There remains a need in the art for nail coatings with enhanced adhesion that do not peel or chip from the nail surface but do not damage or discolor the nail surface.